Spacetime has no physical existence

"In classical general relativity...there is no preferred time variable...[but] we nevertheless have a notion of spacetime for each given solution. [By contrast] in quantum theory there are no trajectories. Thus, THERE IS NO PHYSICAL SPACETIME IN QUANTUM GRAVITY...."

This is the most interesting book of philosophy I can remember seeing in several decades

(also a book of natural philosophy, that is to say, physics)

The online draft of Rovelli's "Quantum Gravity" to be published by Cambridge U. Press

space IS the gravitational field----things move, rotate, accelerate with reference to the gravitational field

time has no physical existence---analogous to the fact that an electron has no real trajectory: the sum over possible trajectories gives the transition amplitude between here and there

time is a property of a classical solution of the GR field----a "trajectory" of geometrical states----but this is only a classical (non-quantum) approximations

there are only transition amplitudes between states of the 3D geometry----development along a classical trajectory does not correspond to physical reality

clocks (whether mechanical, electronic, astronomical) are real physical systems that we distinguish from other systems by convention---they evolve according to sum-over-paths transition amplitudes along with everything else---thus observed states of the world can include readings on various clocks but there is no physical time axis

i am confused by all this, of course there is no physical spacetime, by definition spacetime is where and when things can be physical. if spacetime were physical they would need somewhere and when to exist, in spacetime as it were, it becomes self referential.

Originally posted by steppenwolf i am confused by all this, of course there is no physical spacetime, by definition spacetime is where and when things can be physical. if spacetime were physical they would need somewhere and when to exist, in spacetime as it were, it becomes self referential.

ow my head!

Rovelli has a list of 10 concepts of time-----on page 57.
You might be interested to see where your concept of time
comes on this list. the list is in order of increasing simplicity and the tenth concept is "no time".

That's an interesting choice of words, because defining "physical" seems to require spacetime. For example, if we take our concepts of things, what properties do we attach to objects classified as being physical? In other words, what seperates idealism with physicalism?

Let's put it another way. An object is physical, if (and only if) it has X properties. Traditionally, the view is that an object is physical if it exists in spacetime. So in that sense, it would seem spacetime itself is physical, if only because it is used to define physical things in the first place.

"In classical general relativity...there is no preferred time variable...[but] we nevertheless have a notion of spacetime for each given solution. [By contrast] in quantum theory there are no trajectories. Thus, THERE IS NO PHYSICAL SPACETIME IN QUANTUM GRAVITY...."

This is the most interesting book of philosophy I can remember seeing in several decades

Hi Marcus, thanks for the interesting material. I've just downloaded it and will read it asap.

I think I would like to hear why you think this is important to philosophy.

as you know, Physics was at one time called "Natural Philosophy"
and the content of any discipline (the questions investigated, the kinds of answers sought, etc) CHANGES with time and the boundaries between physics and various branches of philosophy shift. There is no set division.

Rovelli, as well as being a GR-specialist, is a historian of science. (He wears that hat as well.) and his account of the HISTORY of GR
and of the ideas of space and time in European thought is, I find, as interesting as his discussion of the mathematical models. History can be very revealing.

He says that at certain turning points in the history of physics the theoreticians have concerned themselves with serious philosophical issues---Galileo, Decartes, Newton for example
were concerned with the nature of space and time. Likewise in the period 1890-1920 there were a bunch of people like that.
There are times when physicists use philosophical reasoning to push forward where they cant reach solely with equations and formulas. Or so Rovelli is saying.

At other times, he says, physicists have avoid philosophical questioning and are concerned with modifying details of equations, cranking out predictions that can be tested experimentally, and elaborating models without changing their
philosophical perspectives.

What he says suggests to me that we are coming out of a comparatively ad hoc pragmatic era and entering a phase when basic concepts such as time will necessarily be adjusted. But I think you should get your take on this directly from the author.
Maybe I will just shut up and see if anyone finds anything interesting in the book

There should be a link there to the book. Maybe that will]
work better for you.

But the link I gave first should work if you wait a few minutes.
There is a free piece of software Adobe Acrobat which I assume
someone has already installed on your computer and which will
come up and, in a few minutes, process the text of the book and get it up on the screen. Please keep me posted if you try this.
Someone else at PF has already successfully used the link so I believe it is workable.